This invention relates to mufflers for the exhaust of pneumatic tools in general and more particularly for mechanized, jumbo mounted, percussive rock drills employed in the mining industry. It is recognized that such drills produce one of the most critical noise problems throughout the world in the field of mining. Expulsion of compressed air from air drills creates a high noise level which must be lowered to enable the operator of the drill to work in comfort and safety.
Attempts to reduce air-exhaust noises in pneumatic percussion rock drills have been directed primarily to hand held and feed leg drills. Retrofit mufflers with chambers around the drill body and exhaust port, have been devised. Such prior art mufflers are illustrated for example, in U.S. Pat. Nos. 3,255,884 and 3,554,316; and British Pat. No. 329,239. However, in many cases, mufflers of this general class have added excess weight, have not been able to withstand rough handling and have suffered from ice accumulation.
Attempts to retrofit mufflers on mechanized jumbo mounted pneumatic drills have been beset by similar problems. Although the added weight can generally be disregarded, bulk, protruding parts and icing are serious problems and have limited retrofit mufflers on jumbo drills. Since the drills often operate in close quarters any protruding portions or air lines are susceptible to being torn off by accidental contact with rocky walls. In addition the large volume of air passing through greatly increase the icing problem.
Conventional jumbo mounted percussive rock drills operate by compressed air which impells a piston in a cylinder to strike a striker-bar. A drill steel rod, attached to the striker-bar, has a drill bit at the other end which digs into the rock. Means are provided for reciprocating the piston within the cylinder and for rotating the drill bit.
In a mechanized mining operation, the drill is mounted on a carriage plate which rides on a boom that can be orinted in a number of directions. One or more booms may be attached to a jumbo or crawler drilling rig. A chain drive attached to the carriage plate provides for advancement and retraction of the drill and for maintaining constant pressure on the drill bit.
During operation of the drill, the compressed-air driven reciprocating piston moves back and forth about 20 to 60 times per second and compressed air is released to the atmosphere twice for each complete piston cycle. The release of compressed air at high velocity into the relatively calm atmosphere generates excessive noise which corresponds to the release frequency and its hormonics, up to the 15th harmonic. Mixing of the high velocity air with the relatively calm atmospheric air produces intense, broadband noise in the frequency range of 2000 to 12,000 Hz. Adiabatic expansion of the air upon release cools it to temperature as low as-40.degree. C, causing the moisture in the air to crystallize out as ice.